1. Field of the Invention
This invention pertains to bulk preparation of crystalline cubic or zinc blende form of gallium nitride.
2. Background Art
Gallium nitride has been considered a promising material for semicondutor devices since about 1970, especially for the development of diodes for emitting blue light and ultraviolet light. Gallium nitride is a candidate material for optoelectrical applications at such photon energies because it forms a continuous alloy systems InGaN and AlGaN. Other advantageous properties for this material include high mechanical and thermal stability, large piezoelectric constants and the possibility of passivation by forming thin layers of Ga.sub.2 O.sub.3 with a bandgap of 4.2 eV. The spontaneous and piezoelectric polarization in wurtzite materials and the high electron drift velocity of 2.times.10.sup.5 m/s can be used to fabricate high power transistors based on AlGaN/GaN heterostructures. For epitaxial growth of gallium nitride by the non-bulk procedure of molecular beam epitaxy it is necessary to supply nitrogen as a molecular beam. Although epitaxial growth of gallium nitride on sapphire and silicon substrates by gas-source molecular beam epitaxy using ammonia has been reported, gallium nitride does not grow on gallium arsenide when ammonia is used, and the nitridation of gallium arsenide surfaces by more active nitrogen sources is suggested for epitaxial growth of gallium nitride on gallium aresnide.
Growth of gallium nitride on gallium arsenide (001) and on gallium arsenide (111) substrates by gas-source molecular beam epitaxy using dimethylhydrazine, as a nitrogen source, produces gallium nitride of different crystal structures. Hexagonal gallium nitride usually exhibits a characteristic (0002) diffraction in x-ray diffraction which corresponds to the diffraction angle 2.theta. of 34.7.degree.. For epilayers on gallium arsenide (001), only a broad peak is observed at 2.theta. of 40.0.degree.. Any peak attributable to hexagonal gallium nitride is not observed. Assuming the zinc-blende structure to have the same band length as the wurtzite structure, the peak at 2.theta. of 40.0.degree. is assigned to (002) diffractions of the cubic latice. For epilayers on gallium arsenide (111) substrates, a sharp peak is observed at 2.theta. of 34.3.
For the growth of electronic devices it is usually advantageous to have a native substrate in order to eliminate any mismatch between expansion coefficients and crystal lattices. Bulk synthesis of gallium nitride in the normal hexagonal form has been known for many years. Crystals of hexagonal gallium nitride have been grown from the vapor phase at high pressure and temperature, in liquid flux melts, and in supercritical anmmonia using basic mineralizers. Such crystals can be cut up to make native substrates for electronic device growth. However, until this invention, the synthesis of cubic gallium nitride crystals in bulk form had not been reported.